1. Field of the Invention
The present invention relates to a detecting circuit for detecting the abnormality of the frequency of an alternating current (AC) power source and, more particularly, to a frequency abnormality detecting circuit for detecting abnormality through a true-false decision of the count number of an output pulse, which is obtained by the waveform shaping and frequency demultiplying of the AC.
2. Description of the Prior Art
For an integrated circuit (referred to hereunder simply as "IC") whose input and output operate using only binary voltages (for example, +5 V and 0 V) as variables, a digital IC has been developed and applied to various technical fields. Customarily, this digital IC is employed even in the case of a detection of the frequency abnormality of an AC power source. For example, a frequency discriminating circuit, which is composed of retriggerable monostable multivibrators and D-type flip-flops, is disclosed on pp. 416-417 of the first edition of the "Manual for Practical Use of a Digital IC Circuit" published in July 1974 by Radio Technical Corp.
FIG. 1 shows substantially the same circuit as the frequency discriminating circuit shown in FIG. 15.10 on page 417 of the above-referenced publication. The circuit of FIG. 1 comprises an AC power source 1, a waveform shaping circuit 2, retriggerable monostable multivibrators 3 and 4, D-type flip-flops 5 and 6, and an OR gate 7.
The operation of the FIG. 1 circuit will be described referring to FIG. 2.
The monostable multivibrators 3 and 4 have oneshot pulse width .tau.1 and .tau.2, respectively. It is assumed that the relationship of .tau.2&lt;.tau.&lt;.tau.1 is established where the period of the AC power source 1 to be detected is .tau. in a normal state. The multivibrators 3 and 4 are triggered by the rising or leading edge of a signal V2, which is obtained by the waveform shaping of a signal V1 of the AC power source 1. The flip-flops 5 and 6 store the output signals of the multivibrators 3 and 4 in synchronism with the signal V2. After the multivibrators 3 and 4 are triggered once their outputs continue if they are triggered again within each one-shot time. The OR gate 7 performs an OR operation of respective outputs Q and Q of the flip-flops 5 and 6.
As shown in FIG. 2, when a zero-crossing time point T1 of the AC power source 1 becomes EQU T1&lt;.tau.2&lt;.tau.1,
signals P11 and P12 from the multivibrators 3 and 4 remain at H (a high level), and a detection output F takes H at the time point T1 to detect the frequency abnormality of the AC power source.
However, because the conventional frequency abnormality detecting circuit is constructed as mentioned above, the detection accuracy of the circuit is limited to the accuracy (normally about .+-.1%) of each one-shot time of the multivibrators 3 and 4. Particularly, in the detection of the frequency abnormality of a commercial power source, the use of such a conventional detecting circuit becomes impractical, since the detection accuracy of 1% or less than 1% is required. Also, the alteration of a detection value causes a problem that such components as a capacitor or a resistor for determining the time constants of the retriggerable monostable multivibrators must be altered consequently.